Tissue ablation can be performed to remove undesired tissue such as cancer cells, or may involve the modification of tissue without removal, such as to stop electrical signal propagation through tissue in patients with, e.g., a cardiac arrhythmia. Often the ablation is performed by passing energy, such as electrical energy, through one or more electrodes causing the tissue in contact with the electrodes to be heat ablated. Cryo-probes and cryo-catheters are also used to freeze the tissue during cryoablation. Cryoablation is used to treat various diseases and disorders including, for example, liver tumors, actinic keratoses, breast cancer, colorectal cancer, cervical intraepithelial neoplasia, prostate cancer, and atrial fibrillation. Cryosurgery creates an iceball that is easily visualized within a defined zone, but is difficult to control or monitor temperatures at the periphery. Other probes employ the use of energy sources including microwaves, lasers, and high intensity ultrasonic devices that mechanically destroy the tissue.
In thermal ablation, collecting information regarding the three-dimensional thermal profile of tissues in and around a treatment zone is extremely important. In view of the tissue temperature gradients that exist during hypothermia and hyperthermia treatments as a consequence of differences in blood flow and thermal conductivity of tissue, temperature monitoring is crucial at multiple sites in and around the treatment zone. Temperature gradients will be greatest at the boundaries of differential energy absorption, perfusion and conductivity, and thus, knowing the temperature throughout the targeted area including at the tumor margin or proliferating edge would be advantageous.
Typically, to measure probe temperature, thermosensors are placed inside a probe or on an outer surface of the probe to monitor temperature at the tip of the probe, also known as “tip temperature.” Tip temperature is often defined as the temperature within the inside of the probe tip, on the outer surface of the ablation lumen, or at any point along the ablation zone of a device.
During cryoablation, once the probe is inserted into a body of tissue, the position of the probe and thermal sensors within the probe are fixed with respect to that tissue and cannot be moved or displaced so long as that tissue is frozen. The immobility of the thermal sensors within such a probe thus severely limit the ability of the probe to provide detailed and accurate thermal information regarding temperatures at a wide variety of positions within and around the iceball created by a cryoablation procedure. Furthermore, this temperature recording does not reflect the temperature of tissues at the periphery of the iceball near surrounding tissues. For example, to attain a designated temperature within a target tissue, the freeze area extends beyond the periphery of the target and into surrounding non-target tissue. In an argon-based system, this means a large portion of the damaged tissue is outside the target site. In a liquid nitrogen based system, colder isotherms are achieved throughout the iceball to increase cell death and control destruction of the targeted tissue, but over-freeze can damage surrounding non-target tissue.
To monitor tissue temperature surrounding a probe prior to, during, or following a procedure, separate thermal monitoring devices have been inserted adjacent or perpendicular to an ablation probe. Issues in positioning and placing the probes with thermal sensors, however, make temperature measurements throughout an entire ablative zone difficult if not impossible. Without effective temperature monitoring, heat ablation and cryoablation procedures are not able to effectively ablate all of the target tissue and avoid damage to surrounding healthy tissues. For example, cancerous tissue may be left behind where ablation is not achieved at the target site due to unknown temperatures within the tumor and within surrounding healthy tissues.
In one method of heat ablation, radio-frequency ablation (RFA) is delivered and monitored by use of temperature or impedance generators. Temperature-based generators allow probes to become heated at a preset value for a predetermined length of time. Temperatures above 50° C. are required for cell injury. Unfortunately, probe and tissue temperatures may not necessarily correlate. This affects cell destruction as heat sensitivity varies between normal tissue and diseased tissue. Further, blood vessels near a treatment zone serve as a heat sink and make it difficult to control and monitor the target temperature.
Impedance generators are another method of monitoring the ablation process. Typically, impedance levels higher than 200Ω are utilized. Comparison of impedance-monitored and temperature monitored RFA generators have indicated that equivalent ablation zones can be created. As charred tissue adheres to the probe surface, however, an artificial increase in impedance may result, thus limiting the ablation zone. Modifications in the generator and the probe have thus led to the development of ‘wet’ or ‘cool’ RFA. Cool saline irrigation disperses charred tissue, allowing more efficient conductive heat transfer.
As described, the common monitoring that defines effective ablation is the temperature at which the tissue is destroyed (ablated) via mechanisms of physical destruction, necrosis, and/or apoptosis. To achieve effective ablation throughout a target tissue, a desired temperature at or beyond the margin (edge) of a given area must be attained. Currently, accurate and effective monitoring of temperatures at precise locations is extremely difficult requiring the utilization and interpretation of data from independent temperature recording and visualization devices such as, e.g., ultrasound. This often results in improper or inaccurate placement of thermosensors which can result in incomplete or excessive tissue destruction during an ablation procedure. Further, the target tissue for ablation may be located where the placement of additional thermosensors or visualization is not possible, making it difficult to effectively monitor the ablation zone.